[OpenMP] Add stubs for Task affinity API

This patch adds __kmpc_omp_reg_task_with_affinity to register affinity
information for tasks. For now, the affinity information is not used,
and the function always succeeds. This also adds the kmp_task_affinity_info_t
structure to store the task affinity information.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D55026


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@347907 91177308-0d34-0410-b5e6-96231b3b80d8